Method of manufacturing a semiconductor method of manufacturing a thin-film transistor and thin-film transistor

ABSTRACT

A method of manufacturing a semiconductor characterized in that, in polycrystallizing an amorphous silicon thin film formed on a substrate through an annealing process, the amorphous silicon thin film has a plane area of 1000 μm 2  or less. A thin-film transistor characterized by comprising an active silicon film which is formed of a plurality of island-like regions arranged in parallel to each other, the island-like regions being formed of a polycrystal silicon thin film having a plane area of 1000 μm 2  or less. A method of manufacturing a thin-film transistor comprising the steps of:  
     forming an amorphous silicon thin film on a substrate; processing the amorphous silicon thin film into a plurality of island-like regions having a plane area of 1000 μm 2  or less; polycrystallizing an amorphous silicon thin film that forms the island-like regions through an annealing process; and forming a thin-film transistor having at least one of the plurality of island-like regions as an active silicon layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor formed of apolycrystal silicon thin film, which is formed by crystallizing anamorphous silicon thin film on an insulator, and a thin-film transistorusing that semiconductor.

[0003] 2. Description of the Related Art

[0004] In recent years, a technique has been popularly researched wherean amorphous silicon thin film is formed on an insulator such as aquartz substrate, allowed to grow into a solid-phase crystal (SPC)through a heating treatment or an annealing process due to theirradiation of a laser beam or an intense light to form a polycrystalsilicon thin film.

[0005] A conventional general technique for obtaining a polycrystalsilicon thin-film by allowing an amorphous silicon thin film to grow ina solid phase will be described below.

[0006] First, an amorphous silicon thin film having a thickness of 500to 5000 Å is formed on a quartz substrate.

[0007] Thereafter, the amorphous silicon thin film thus formed is heatedto 400 to 1100° C. and subjected to an annealing process to make theamorphous silicon thin film grow into crystal. In this situation, aheater, infrared rays or the like is used as heating means.

[0008] The annealing process may be conducted by the irradiation of alaser beam or an intense light other than heating.

[0009] In the above-mentioned manner, a polycrystal silicon thin film isobtained.

[0010] The polycrystal silicon thin film thus obtained is used as anactive silicon layer of the thin-film transistor (TFT), to therebyprovide a thin-film transistor. As a result, a liquid-crystal displayunit, image sensor or the like which is high in operation speed and inimage quality is obtained using the above thin film transistor.

[0011] Up to now, the polycrystal silicon thin film obtained byannealing the amorphous silicon thin film has difficulty in lowering adefect density in crystal.

[0012] The thin-film transistor using the polycrystal silicon thin filmthus formed as the active silicon film is hindered from realizing animprovement in various characteristics of the thin-film transistor, forexample, the lowering of a threshold voltage (V_(th)), an increase inmobility, a decrease in a leak current (I_(off)), etc., because thedefect density in the active silicon layer is high.

SUMMARY OF THE INVENTION

[0013] The present invention has been made in view of the abovecircumstances, and therefore an object of this invention is to make apolycrystal silicon thin film obtained by annealing an amorphous siliconthin film low in defect density and high in quality.

[0014] Another object of the invention is to provide a thin-filmtransistor using a polycrystal silicon thin film obtained through anannealing process with a lowered threshold voltage (V_(th)) and leakcurrent (I_(off)) and an increased mobility.

[0015] In order to solve the above-mentioned problem, according to oneaspect of the present invention, there is provided a method ofmanufacturing a semiconductor characterized in that, inpolycrystallizing an amorphous silicon thin film formed on a substratethrough an annealing process, said amorphous silicon thin film has aplane area of 1000 m² or less.

[0016] In the above-mentioned method, the amorphous silicon thin film ispreferably 1000 Å or more, more preferably 2000 Å to 10000 Å inthickness.

[0017] Also, according to another aspect of the present invention, thereis provided a thin-film transistor having an active silicon film whichis formed of a plurality of island-like regions arranged in parallel toeach other, said island-like regions being formed of a polycrystalsilicon thin film having a plane area of 1000 μm² or less.

[0018] In the thin-film transistor thus constituted, the island-likeregions are formed of a polycrystal silicon thin film which ispreferably 1000 Å or more, more preferably 2000 Å to 10000 Å inthickness.

[0019] Also, according to another aspect of the present invention, thereis provided a method of manufacturing a thin-film transistor comprisingthe steps of: forming an amorphous silicon thin film on a substrate;processing said amorphous silicon thin film into a plurality ofisland-like regions having a plane area of 1000 μm² or less;polycrystallizing an amorphous silicon thin film that forms saidisland-like regions through an annealing process; and forming athin-film transistor having at least one of said plurality ofisland-like regions as an active silicon layer.

[0020] In the above-mentioned method, the amorphous silicon thin film ispreferably 1000 Å or more, more preferably 2000 Å to 10000 Å inthickness.

[0021] The present applicant has found that after the amorphous siliconthin film has been formed as island-like regions that have a plane area(an area viewed from an upper surface of the substrate) of 1000 μm² orless, it is subjected to an annealing process by heating or theirradiation of a laser light beam or an intense light to form apolycrystal silicon thin film with the result that a polycrystal siliconthin film which is low in defect density and high in quality isobtained.

[0022]FIG. 1 is a graph showing a relationship between a thresholdvoltage (V_(th)) and an area of the island-like regions of thepolycrystal silicon thin-film transistor, in which the island-likeregions are 1250 Å in thickness.

[0023] As shown in FIG. 1, it has been found that as the area of theisland-like regions is small, the threshold voltage approaches 0 (zero)more in both of a p-channel and an n-channel so that the defect densityis lowered.

[0024] In FIG. 1, there has been found that an extremely excellentcrystalline property is obtained when the plane area of the island-likeregions is 1000 μm² or less.

[0025] Also, when the plane area of the island-like regions is 1000 μm²or less, the island-like regions may be of a square, a rectangle orother shapes.

[0026] Further, when the island-like regions are 1 μm or more in planearea, the semiconductor is satisfactorily available as a device, andalso it can be readily manufactured by a usual technique.

[0027] On the other hand, in the case where the polycrystal silicon thinfilm is provided as the active silicon layer of the thin-filmtransistor, because an area of the island-like regions is limited insize, the thin-film transistor using that polycrystal thin film is alsolimited in size, which causes a limit to the performance of thethin-film transistor.

[0028] Under the above circumstances, the applicant has found that aplurality of island-like regions a plane area of which is 1000 μm² orless are arranged in parallel to each other as active silicon layersthat constitute the source region, the drain region and the channelformation region of a thin-film transistor so as to increase asubstantial channel width, thereby being capable of obtaining apolycrystal thin-film transistor which allows a sufficient amount ofcurrent to flow therein, which has the channel formation region low indefect density, and which is high in performance.

[0029]FIG. 3 shows an example of a plane shape of a thin-film transistorusing a plurality of island-like regions as an active silicon layer.

[0030] In FIG. 3, a plurality of island-like regions 301 are arranged inparallel to each other to form an active silicon layer 305 of athin-film transistor. On the island-like regions 301 are disposed a gateelectrode 302, a source electrode 303 and a drain electrode 304 to formone thin-film transistor. Appropriate intervals between the respectiveisland regions is several to several tens μm. As the intervals aresmall, the plane area of the active silicon layer can be reduced more.

[0031] In the island-like region, as its plane area is reduced, thedefect density is reduced more in a polycrystallized state, therebybeing capable of reducing a leak current.

[0032] Also, the applicant has found that the amorphous silicon thinfilm is set to preferably 1000 Å or more, more preferably 2000 Å to10000 Å in thickness, thereby lowering the defect density of thepolycrystal silicon thin film obtained by crystallizing that amorphoussilicon thin film.

[0033]FIG. 2 is a graph showing a relationship between the defectdensity of a polycrystal silicon thin film in a solid-phase growth andthe thickness of an initial amorphous silicon thin film. In this case, atemperature of a solid-phase crystal (SPC) is 600° C.

[0034] It has been found from FIG. 2 that as the thickness is made thin,the defect density is reduced more.

[0035] However, in crystallizing the thick initial amorphous siliconthin film through an annealing process, a stress of about 3×10⁻⁹ dyn/cm²due to a phase change is developed with the result that the polycrystalsilicon thin film formed may be cracked.

[0036] Accordingly, when the polycrystal silicon thin film formed bycrystallizing the thin amorphous silicon thin film is used as an activesilicon layer that constitutes the channel formation region of thethin-film transistor as it is, this may cause the defect of the deviceor the lowering of the performance.

[0037] However, the applicant has found that even though the thicknessof the amorphous silicon thin film is 1000 Å or more, more particularly2000 Å to 10000 Å, if the island-like regions formed of the amorphoussilicon thin film are set to 1000 μm² or less in area and then annealedfor crystallization, a polycrystal silicon thin film is obtained whichhas no cracks and is lower in the defect density.

[0038] Also, when the thickness of the amorphous silicon thin filmbecomes thicker than 10000 Å, cracks are liable to occur.

[0039] According to the present invention, there can be obtained apolycrystal thin-film transistor that allows a sufficient amount ofcurrent to flow therein, has a channel formation region which is low indefect density, and is high in performance.

[0040] The thin-film transistor like this can reduce power consumptionbecause a threshold voltage (V_(th)) and a leak current (I_(off)) arelowered. Also, the thin-film transistor can operate at a high speed andallow a large current to flow therein because the mobility (μ) isincreased.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The above and other objects and features of the present inventionwill be more apparent from the following description taken inconjunction with the accompanying drawings.

[0042]FIG. 1 is a graph showing a relationship between a thresholdvoltage (V_(th)) and an area of island-like regions of a polycrystalsilicon thin-film transistor;

[0043]FIG. 2 is a graph showing a relationship between the defectdensity of a polycrystal silicon thin film in a solid-phase growth andthe thickness of an initial amorphous silicon thin film;

[0044]FIG. 3 is a diagram showing an example of a plane shape of athin-film transistor using a plurality of island-like regions as anactive silicon layer;

[0045]FIGS. 4A to 4D are diagrams showing a process of manufacturing asemiconductor in accordance with a first embodiment; and

[0046]FIGS. 5A to 5D are diagrams showing the upper surfaces of thesemiconductor shown in FIGS. 4A to 4D.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] Now, a description will be given in detail of embodiments of thepresent invention.

[0048] (First Embodiment)

[0049] A first embodiment shows an example in which an active matrixcircuit and a peripheral drive circuit both of which are formed ofpolycrystal silicon thin-film transistors are formed on the samesubstrate.

[0050]FIGS. 4A to 4D show a process of manufacturing the semiconductorin accordance with the first embodiment.

[0051]FIGS. 5A to 5D show the upper surfaces of the semiconductor incorrespondence with FIGS. 4A to 4D, respectively. FIGS. 5A to 5D arediagrams viewed from the upper portions of FIGS. 4A to 4D. FIGS. 4A to4D are cross-sectional views taken along a line A-A′ of FIGS. 5A to 5D,respectively.

[0052] In FIGS. 4A to 4D, quartz was first used for a substrate 401.Instead, a glass substrate such as Corning Corp. 7059 may be used.

[0053] The substrate 401 is cleaned, and a silicon oxide under film 402having a thickness of 2000 Å is formed through the plasma CVD techniqueusing TEOS (tetra ethoxy silane) and oxygen as a raw gas.

[0054] Then, an initial amorphous silicon thin film having a thicknessof 1000 Å or more, preferably 2000 Å to 10000 Å, in this example 3000 Åis formed by the plasma CVD technique.

[0055] Subsequently, the initial amorphous silicon thin film ispatterned through the dry etching in such a manner that island-likeregions forming active silicon layers 403 to 405 are disposed atpositions where thin-film transistors of an active matrix portion and aperipheral drive circuit portion are formed (FIG. 4A).

[0056] As shown in FIG. 5A, there are formed island-like regions 501 to507 formed of amorphous silicon thin films to thereby form activesilicon layers 403 to 405.

[0057] The size of the respective island-like regions is 20 μm×50 μm(width×length) in this example so that the area of the plane shape isset to 1000 μm² or less.

[0058] In the peripheral drive circuit portion requiring a high-speeddrive, three island-like regions are provided for one thin-filmtransistor, and in the active matrix portion demanding a reduced leakcurrent, one island-like region is provided for one thin-filmtransistor. It is needless to say that the number of island-like regionsmay be increased in accordance with a required standard.

[0059] In this example, the respective intervals between the adjacentisland-like regions that form one thin-film transistor of the peripheraldrive circuit portion was set to 4 μm.

[0060] Also, in the thin-film transistor of the active matrix portion,the active silicon layer 405 is formed by one island-like region in thisexample, however, it is needless to say that it may be formed by aplurality of island-like regions.

[0061] Also, the active silicon layer 405 may be formed by a pluralityof island-like regions each having a smaller plane area. In this case,the defect density is lowered more so that the leak current can belowered.

[0062] Further, the shape of the island-like regions forming a thin-filmtransistor may be different between the active matrix portion and theperipheral drive circuit portion.

[0063] Then, the island-like regions formed of those amorphous siliconthin films are crystallized through the annealing process.

[0064] A substrate temperature was set to 500° C. to 1100° C., in thisexample 700° C., and a heating time was set to 2 hours to 72 hours, inthis example 48 hours.

[0065] The annealing process may be conducted by the irradiation of alaser beam or an intense light (infrared rays or the like) other thanheating.

[0066] Through that crystallizing process, the island-like regions 501to 507 were formed into a polycrystal silicon thin film which has beencrystallized excellently.

[0067] Thereafter, through the plasma CVD technique, a silicon oxidefilm 407 that functions as a gate insulating film was formed at athickness of 1500 Å.

[0068] On the silicon oxide film 407, an aluminum film 6000 Å inthickness is formed through the sputtering technique and then patternedby etching, thereby forming gate electrodes 407 to 409.

[0069] Subsequently, the active silicon layers 403 to 405 were dopedwith impurities giving an n-type conduction type or a p-type conductiontype in a self-matching manner with a mask of the gate electrodes 407 to409 through the ion doping technique.

[0070] In this example, as the doping gas, phosphine (PH₃) was used forthe n-type doping, and diborane (B₂H₆) was for the p-type doping.

[0071] In this example, the thin-film transistor in the pixel region wasmade the p-channel type. In other words, the active silicon layers 404and 405 were doped with the p-type impurities, and the active siliconlayer 403 was doped with the n-type impurities.

[0072] As a result, there can be formed p-type impurity regions 413,415, 416 and 418, n-type impurity regions 410 and 412, andsubstantially-intrinsic channel formation regions 411, 414 and 417.

[0073] Thereafter, an annealing process was conducted on those regionsat 400° C. to 800° C. for 1 to 12 hours, typically at 600° C. for twohours so that the doped impurities are activated (FIG. 4B).

[0074] Shown in FIG. 5B is that in the respective active silicon layers403 and 404, the gate electrodes 407 and 408 are disposed on a pluralityof island-like regions.

[0075] Then, an insulating film consisting of a silicon nitride 500 Å inthickness and a silicon oxide 3000 Å in thickness was formed as a firstinterlayer insulator 419 thereon through the plasma CVD technique.

[0076] Subsequently, contact holes 420 to 424 are formed in the firstinterlayer insulator 419, and electrodes/wirings 425 to 428 were formedusing a multi-layer film made of metal material, for example, titan 500Å in thickness and aluminum 4000 Å in thickness (FIGS. 4C and 5C).

[0077] In the first embodiment, one of the contact holes 420 to 423 ofthe active silicon layers 403 and 404 is formed for three island-likeregions as shown in FIG. 5C. However, one contact hole may be formed foreach island-like region.

[0078] Thereafter, a silicon oxide thin film 4000 Å in thickness isfurther formed thereon through the plasma CVD technique as a secondinterlayer insulator 429.

[0079] Then, a contact hole 430 was formed in the impurity region at theactive matrix region side where the pixel electrode of a thin-filmtransistor is formed, and an ITO (indium tin oxide) film 800 Å inthickness was formed thereon. That film was so etched as to form a pixelelectrode 431 (FIGS. 4D and 5D).

[0080] In that way, the active matrix portion and the peripheral drivecircuit portion can be formed on the same substrate.

[0081] The active matrix circuit and the peripheral drive circuit thusformed is excellent because it is small in leak current (I_(off)), lowin power consumption, and high in operation speed.

[0082] That substrate and an opposing substrate having an electrode onone surface thereof are located through liquid crystal, thereby beingcapable of manufacturing a liquid-crystal electro-optical device.

[0083] As was described above, according to the present invention, apolycrystal thin-film transistor can be obtained which allows asufficient amount of current to flow therein, which has the channelformation region low in defect density, and which is high inperformance.

[0084] The thin-film transistor like this allows the threshold voltage(V_(th)) and the leak current (I_(off)) to be lowered, the powerconsumption can be lowered. Also, because the mobility (μ) is increased,the thin-film transistor can operate at a high speed and allow a largecurrent to flow therein.

[0085] The foregoing description of a preferred embodiment of theinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed, and modifications andvariations are possible in light of the above teachings or may beacquired from practice of the invention. The embodiment was chosen anddescribed in order to explain the principles of the invention and itspractical application to enable one skilled in the art to utilize theinvention in various embodiments and with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

1-15. (canceled)
 16. An active matrix circuit comprising: asemiconductor layer; a p-type impurity region provided in saidsemiconductor layer; and an interlayer insulating film comprisingsilicon nitride provided over said semiconductor layer.
 17. A circuitaccording to claim 16 wherein said active matrix circuit is incorporatedinto a liquid-crystal display.
 18. A circuit according to claim 16wherein said active matrix circuit is incorporated into an image sensor.19. A circuit according to claim 16 wherein said active matrix circuitis incorporated into a liquid-crystal electro-optical device.
 20. Acircuit according to claim 16 wherein said semiconductor layer comprisesa crystal silicon.
 21. An active matrix circuit comprising: asemiconductor layer; a p-type impurity region provided in saidsemiconductor layer; and an interlayer insulating film comprising asilicon nitride layer and a silicon oxide layer, said interlayerinsulating film provided over said semiconductor layer.
 22. A circuitaccording to claim 21 wherein said active matrix circuit is incorporatedinto a liquid-crystal display.
 23. A circuit according to claim 21wherein said active matrix circuit is incorporated into an image sensor.24. A circuit according to claim 21 wherein said active matrix circuitis incorporated into a liquid-crystal electro-optical device
 25. Acircuit according to claim 21 wherein said semiconductor layer comprisesa crystal silicon.
 26. An active matrix circuit comprising: asemiconductor layer; a p-type impurity region provided in saidsemiconductor layer; and a conductive layer comprising titanium andaluminum over said interlayer insulating film.
 27. A circuit accordingto claim 26 wherein said conductive layer comprises an electrode.
 28. Acircuit according to claim 26 wherein said conductive layer comprises awiring.
 29. A circuit according to claim 26 wherein said active matrixcircuit is incorporated into a liquid-crystal display.
 30. A circuitaccording to claim 26 wherein said active matrix circuit is incorporatedinto an image sensor.
 31. A circuit according to claim 26 wherein saidactive matrix circuit is incorporated into a liquid-crystalelectro-optical device.
 32. A circuit according to claim 26 wherein saidsemiconductor layer comprises a crystal silicon.
 33. A circuit accordingto claim 26 wherein said titanium and said aluminum are formed in amulti-layer film.